Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers, having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ a fixing roller and a pressing roller pressed against the fixing roller to form a nip therebetween through which the recording medium bearing the toner image passes. The fixing roller and the pressing roller are heated by a fixing roller heater and a pressing roller heater disposed inside or outside the fixing roller and the pressing roller, respectively. As the recording medium bearing the toner image passes through the nip, the fixing roller heated by the fixing roller heater and the pressing roller heated by the pressing roller heater together apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium.
Such fixing device may enter a standby mode after the recording medium bearing the fixed toner image is discharged from the nip, in which at least one of the fixing roller heater and the pressing roller heater is turned on for a predetermined time period (e.g., 60 seconds) so that the fixing roller and the pressing roller are heated to a desired fixing temperature quickly upon receipt of the next job. After the predetermined time period elapses in the standby mode without receiving the next job, the at least one of the fixing roller heater and the pressing roller heater is turned off so that neither the fixing roller heater nor the pressing roller heater heats the fixing roller and the pressing roller.
However, in a case where the fixing roller and the pressing roller should be turned off immediately after the previous job is finished, for example, if the fixing device does not receive the next job within the predetermined period of time, power is wasted for keeping one of the heaters on throughout the standby mode.
To address this problem, two control methods are proposed. The first method is to turn off the heater before the last recording medium of the job is discharged from the nip. This method is employed to prevent overheating of the fixing roller after the job due to absence of the recording medium that draws heat from the fixing roller. Accordingly, this method is effective with a heater disposed inside the fixing roller because heat conduction from the inner surface to the outer surface of the fixing roller overheats the fixing roller. That is, in this method, the heater is turned off for a substantial period of time even in the standby mode to prevent overheating of the fixing roller, resulting in power saving.
By contrast, if the fixing device has an induction heater disposed outside the fixing roller, the temperature of the outer surface of the fixing roller is higher than that of the inner surface of the fixing roller, causing no heat conduction from the inner surface to the outer surface thereof that overheats the fixing roller. Accordingly, the induction heater, which is turned off before the recording medium of the job is discharged from the nip, needs to be turned on again immediately after the fixing device enters the standby mode, wasting power.
The second method is to lower the target temperature of the fixing roller and the pressing roller after the job is finished. For example, a target temperature of the fixing roller in the standby mode is lower than a target temperature during the job. Similarly, a target temperature of the pressing roller in the standby mode is lower than a target temperature during the job, with the temperature differential therebetween greater than that between the target temperatures of the fixing roller. However, with this method, the heaters are not turned off in the standby mode. As a result, power saving is insufficient.